Abstract
This paper addresses a multiscale approach for heat recovery systems, used in two distinct applications. In both applications, a microscale approach is used (microchannel heat sinks and heat pipes) for macroscale applications (cooling of a photovoltaic—PV cell), and the thermal energy of exhaust gases of an internal combustion engine is used for thermoelectric generators with variable conductance heat pipes. Several experimental techniques are combined such as visualization, thermography with high spatial and temporal resolution, and the characterization of the flow hydrodynamics, such as the friction losses. The analysis performed evidences the relevance of looking at the physics of the observed phenomena to optimize the heat sink geometry. For instance, the results based on the dissipated heat flux and the convective heat transfer coefficients obtained in the tests of the microchannel-based heat sinks for cooling applications in PV cells show an improvement in the dissipated power at the expense of controlled pumping power, for the best performing geometries. Simple geometries based on these results were then used as inputs in a genetic algorithm to produce the optimized geometries. In both applications, the analysis performed evidences the potential of using two-phase flows. However, instabilities at the microscale must be accurately addressed to take advantage of liquid phase change. In this context, the use of enhanced interfaces may significantly contribute to the resolution of the instability issues as they are able to control bubble dynamics. Such an approach is also addressed here.
Highlights
The concept of heat recovery systems is emerging in the design of internal combustion engines (ICE), for heavy-duty vehicles, where different waste heat recovery techniques have been explored [1,2,3,4,5]
After a basic experimental approach to study the effect of main geometric parameters of straight-line microchannel heat sinks, main geometric parameters are introduced in a genetic algorithm to achieve the optimized geometry for the specific working conditions of the used PV cells
After fabricating and characterizing the microchannel heat sinks, several experimental tests were carried out according to the following procedure: (1) The power supply was connected, with a fixed current of 5 A, providing a constant heat flux in all tests of 1386.83 W/m2; (2) The syringe was filled with the cooling fluid with the aid of a tube connected to another syringe so that the process would be as fast as possible; (3) The temperature of the syringe heating sleeve was adjusted, adjusting the temperature of the necessary fluid for the various planned tests; (4) The desired flow rate for the test at the syringe pump was adjusted; (5) Before running the experiment, the temperature of the steel should be stabilized
Summary
The concept of heat recovery systems is emerging in the design of internal combustion engines (ICE), for heavy-duty vehicles, where different waste heat recovery techniques have been explored [1,2,3,4,5]. Enhanced surfaces can have a high potential in overcoming this obstacle; artificial cavities should be implemented to boost nucleation, creating a bigger contact angle for the bubble with the heated surface that would lead to more nucleation sites, and conceiving a surface where the cross-sectional area of the channel would tend to increase along the flow might all be an option In this context, this second case study addresses an optimization study of a heat sink based on microchannels to cool small PV cells. After a basic experimental approach to study the effect of main geometric parameters of straight-line microchannel heat sinks, main geometric parameters are introduced in a genetic algorithm to achieve the optimized geometry for the specific working conditions of the used PV cells Stochastic methods such as the genetic algorithms are pointed to have several advantages when compared to the so-called deterministic methods [18,19,20], namely that they depict a degree of randomness that allows for a full domain search, escaping local peaks. The approach followed and the results obtained are extensively described and discussed in an extended version of a previous publication [21]
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